Structural effects on SAPO-34 and ZIF-8 materials exposed to seawater solutions, and their potential as desalination membranes

Abstract

The effects of saline water on the material properties of two promising salt rejecting structures, SAPO-34 and ZIF-8, were explored for desalination. Immersing SAPO-34 in seawater revealed a release of Al which involved some replacement by Ca, K, Mg and Na. X-ray powder diffraction revealed a robust structure in deionised and sea water solutions. ZIF-8 on the other hand, lost 1% of its mass due to release of Zn into both deionised water and seawater solutions, yet structure remained intact. N2 and positron annihilation lifetime spectroscopy porosimetry techniques revealed loss of surface area and mesopores on both materials, explained by capillary forces acting to close them. Membrane reverse osmosis revealed no liquid water flux for SAPO-34 due to the intactness of the membrane and very small pore size which contributed to prohibitive resistance to liquid water flow. ZIF-8 membranes permeated water, but did not demonstrate significant salt rejection indicating that either the slight mass loss impacted membrane integrity to reject salt, or that liquid water diffusion in ZIF-8 does not follow a salt rejecting path. SAPO-34 and ZIF-8 structures are stable structures for aqueous applications including sea salts, but further work is needed to develop them for reverse osmosis desalination.

A method for defect repair of MFI-type zeolite membranes by multivalent ion infiltration

“…desalination). SAPO-34 (a silicoaluminophosphate with chabazite (CHA) type framework) and ZIF-8 (zeolitic imidazolate framework) have been explored for RO desalination but did not possess the correct structure or integrity to permeate water and reject salts [3]. However inorganic materials which possess these properties are silica [4,5], hybrid organically bridged silica [6,7] and zeolites [8][9][10][11][12][13].…”

Preparation of nano‐ZIF‐8 in methanol with high yield

“…Reactions were often carried out above 100 °C under self‐generated pressure or under 100 °C with stirring. Reaction time was mostly in the range of 4–6 h or from 1–3 d …”

Improving hydrostability of ZIF-8 membranes via surface ligand exchange

“…Duke and co-workers [33] explored the potential of tubular alumina tube supported ZIF-8 membranes for saline water desalination via reverse osmosis. They reported an extremely high flux of 600 kg/m 2 .h, but a low salt rejection of 6% possibly due to the membrane defects.…”